Stock Enhancement in Greenlip Abalone Part II: Population and Ecological Effects

Abstract

A series of stock enhancement experiments were carried out on Haliotis laevigata populations. Methodologies included a large-scale BACI (before, after, control, impact) experiment (42 sites); a carrying capacity experiment, which involved a high-density release at two sites; and a detailed survey of abalone populations and ecological parameters. Increased densities were detected for most age classes, although fishing mortality began obscuring the effect by age 5+. Age-4+ animals showed the clearest result, with no difference between enhanced and control sites at 6, 12, and 18 months post-release, and then a 300% increase at enhanced sites at 30 months post-release. Overall, a single release of age-1+ animals in May 2006 had doubled the total density by November 2008. In the carrying capacity experiment, densities initially increased rapidly (by up to 800%) but had stabilized at a 400% increase after 2.5 years, at around 8 per m2. This was the predicted carrying capacity, with the enhanced cohort representing 50% of the population. A PERMANOVA (permutational multivariate analysis of variance) analysis of ecological similarity detected no effect of enhancement, although changes in algal percent of coverage were detected at both control and enhanced sites. Overall, this study suggests that as long as release densities are controlled within natural limits, successful stock enhancement can be attained for this species with minimal ecological impacts.